1. Field of the Invention
The present invention relates generally to the transmission of analog audio tone based communications, and more particularly to such transmission over a packet data network.
2. Related Art
Many older ‘legacy’ telecommunications systems use dial-up telephone lines with analog modems to send data between equipment and digital computer systems. The modulator of a modulator/demodulator (modem) at one end of the telephone line converts digital data into an audio tone sequence that can be transmitted within the audio band of the standard telephone channel. At the other end of the line the demodulator of a second modem may decode the signal and convert it back into digital data.
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted. This is done in a similar fashion to a musician modulating a tone (a periodic waveform) from a musical instrument by varying its volume, timing and pitch. The three key parameters of a periodic waveform are its amplitude (“volume”), its phase (“timing”) and its frequency (“pitch”). Any of these properties can be modified in accordance with a low frequency signal to obtain the modulated signal. Typically a high-frequency sinusoid waveform is used as carrier signal, but a square wave pulse train may also be used.
In telecommunications, modulation may include the process of conveying a message signal, for example, a digital bit stream or an analog audio signal, inside, e.g., another signal that can be physically transmitted. Modulation of a sine waveform may be used to transform a baseband message signal into a passband signal, for example a low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited passband frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the center frequency (typically the carrier frequency) of the passband.
A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (from “modulator-demodulator”).
There are many standard modem protocols for communicating in this way over analog telephone lines. These standard protocols transfer information at speeds ranging from less than 75 to 56,000 bits/second or more. Typically the lower speed modems use Frequency Shift Keying (FSK) and are used by older equipment or in cases where the quality of the analog line is very poor. Higher speed modems also use Phase Shift Keying (PSK) or Quadrature Amplitude Modulation (QAM) but depend on a high quality audio connection and are much more susceptible to noise and small disturbances on the line than is FSK.
Because the quality of dial up lines still varies enormously, there are important legacy applications still in widespread use today for which a very low data rate modem is automatically chosen in order to minimize the possibility of errors during transmission.
For example, facsimile machines typically attempt to operate at a maximum speed of 14.4 Kbps, and start their communication by testing the quality of the line. In many cases they will negotiate a final transmission speed of 9,600 bps or slower. As a second example, many Point of Sale (POS) terminals and Automatic Teller Machines (ATM) always connect at the relatively slow speed of 2400 bps regardless of the quality of the connection. By connecting at low speed, the chance of errors that might cause a failure mid way through the transaction are minimized. A third example of low data rate legacy applications is alarm systems that use Dual Tone Multiple Frequency bursts of audio to send data at a data rate of less than 50 bits/second, but remain very resilient to line disturbances and noise. Alarm systems also sometimes use pulse modulation which operates by turning an audio carrier signal on or off at a very low rate, typically between 10 and 40 times a second.
Regardless of the data rate being used, all modems described above are designed to operate in real time when directly connected over the public switched telephone network (PSTN). In some cases the modems generate and receive a constant audio stream with no breaks in the signal. They are designed to operate within predefined line-quality and signal quality limits that are not always achieved. At the receiving end of a modem link any break in signal or short-term distortion in signal quality may result in data errors from which the application being run over the link may not recover.
A common requirement in telecommunications systems is to replace the dial up connection with an alternative connection, such as a wireless or terrestrial data packet network, without disturbing the equipment at either end of the dial up link. For example, it may be desirable to connect an existing alarm system to a cellular wireless data IP network in order to provide service in an area where landlines are unreliable. Such an application may be for use as the primary link or as an emergency backup to the dial network. In such cases it may be desirable to avoid the expense of upgrading the existing system to one that is “IP ready” but rather to use the IP network to transparently connect the two ends of the link. Often a modem is embedded in an expensive end device and cannot be bypassed without replacing the whole unit, for example in an ATM machine or in a Commercial HVAC system. At the other end of the link, access to the Legacy host or server system may also only be through dial-in lines. It is desirable under these circumstances for the legacy equipment at both ends of the link to operate substantially as each had previously with the PSTN line attached and that users of the equipment would experience no major changes in operation.
The general literature describes two conventional methods for attaching dial modems to packet data networks so that they may operate transparently to the attached equipment. These are known as Modem Passthrough and Modem Relay.
In the first method, modem passthrough, the analog signal from the modems at each end of the link is converted directly into a digital stream using an analog to digital converter at the analog input of a packet router or gateway. This digital representation of the audio signal normally takes the form of Pulse Code Modulation (PCM) and is passed over the network to the other end of the link in packet form without any decoding. Using modem passthrough, the receiving end of the link, the signal is regenerated exactly as it was received from the sending modem as if the two modems were still directly connected. The audio stream thereby generated normally conforms to the G.711 standard and has a data rate of 64 Kbps. G.711 is the digital audio standard used by the digital hierarchy of the PSTN, sometimes known as full rate voice, or toll quality voice. In this mode of operation the packet network effectively performs exactly the same functions as a standard digital telephone network, the primary difference being that the packet network introduces a substantial transmission delay that varies from packet to packet. This variation in delay is called jitter. Buffers in the end-point routers may absorb jitter to some extent, but there is always a trade-off between the magnitude of jitter that can be absorbed and the delay that can be tolerated by the applications running over the network. For many applications the delay and jitter of an IP network can exceed the operational response and timeout limitations of the attached systems and modem passthrough will not operate successfully, even for low data rate modems. Additionally, any significant packet loss during the call is likely to be fatal to the transmission. The problem is particularly acute where the packet network experiences relatively long delays, such as over satellite links, or where there is a wide variation in delay and a possibility of packet loss, such as with some wireless data services.
A common method of using modem passthrough is to connect an analog modem to an analog voice port on an IP router. IP routers designed to carry voice traffic using Voice over IP standards (VoIP) often work in the mode described above, where the incoming analog signal is converted directly to a G.711 compatible digital stream without any further processing. However many VoIP devices also compress the G.711 data to minimize the amount of data sent over the packet network, which may typically reduce the delay and jitter experienced. G.723, G.726, G.728, G.729 are examples of voice compression standards that are commonly used in VoIP systems, but there are a large number of other algorithms and standards in use. Using such voice compression techniques the data rate is reduced from full rate to a fraction. Many commonly used algorithms operate at an eighth rate or less. G.729 Annex B, for example, operates at 8 Kbps. All commonly used VoIP compression algorithms function by discarding information during the compression process with the result that the audio signal output at the receiving end of the link is a distortion of the input audio signal to some extent. The algorithms are structured to retain those qualities that optimize voice recognition by the human ear and not to retain the exact frequency and phase qualities of the incoming signal. As a result, the decoders of analog modems normally cannot decode signals that have been transmitted over VoIP systems that use compression. Even the very slowest DTMF and FSK modems do not operate when an attempt is made to use Modem Passthrough over a VoIP network with voice compression enabled.
In the second conventional method, modem relay maybe used to transfer dial modem traffic over a packet network. With Modem Relay the modem output is demodulated at the local router or gateway and is converted back to the original digital data before transmitting over the packet network. At the receiving router or gateway, the process is reversed and the modem signal is recreated converting the data stream back into the modem format required by the host or server system. A benefit of modem relay is the reduced amount of data that needs to be sent over the packet network. For this reason alone, modem relay systems are less likely to suffer from lost packets, which increase the likelihood of a transaction being successful. Nevertheless, the operational response and timeout limits of an application may cause a Modem Relay communication to fail, simply due to the unavoidable delay and jitter introduced by the use of the packet data network.
The wide deployment of embedded dial modems and similar communications devices that rely on tone based communications over PSTN lines makes it desirable to find an alternative method of connection that is easily deployed and more dependable. IP networks, and cellular wireless data services in particular, are the most widely deployed with very competitive pricing for relatively low bandwidth applications such as those described above. They are also ideal for emergency and backup applications. However IP networks, and cellular wireless data networks in particular, are not geared to support the transportation of frequency and phase sensitive communications signals or to supporting an analog system with critical timing windows, with the result that the conventional techniques of modem passthrough and modem relay do not work successfully for many such applications.
An alternative solution is needed to overcome shortcomings of conventional solutions.